UVa 10375 - Choose and divide（唯一分解定理）

 1 import java.io.*;
 2 import java.util.*;
 3 
 4 public class Main {
 5     static int e[];
 6     static ArrayList<Integer> prime = new ArrayList<Integer>();
 7     
 8     static void getPrimeNumber(int up) { // 获取素数
 9         boolean isp[] = new boolean[up+5];
10         Arrays.fill(isp, true);
11         int u = (int)Math.sqrt(up + 0.5);
12         for(int t = 2; t <= u; t++) if(isp[t]) {
13             for(int i = t * t; i <= up; i += t) isp[i] = false;
14         }
15         for(int i = 2; i <= up; i++) if(isp[i]) prime.add(i);
16     }
17     
18     static void addInteger(int n, int d) { // 乘以或除以n. d=1表示乘，d=-1表示除
19         for(int i = 0; i < prime.size(); i++) {
20             while(n % prime.get(i) == 0) {
21                 n /= prime.get(i);
22                 e[i] += d;
23             }
24             if(n == 1) break;
25         }
26     }
27     
28     static void addFactorial(int n, int d) { // 把结果乘以(n!)的d次方
29         for(int i = 1; i <= n; i++) addInteger(i, d);
30     }
31     
32     public static void main(String args[]) {
33         Scanner cin = new Scanner(new BufferedInputStream(System.in));
34         getPrimeNumber(10000);
35         e = new int[prime.size()];
36         
37         while(cin.hasNext()) {
38             int p = cin.nextInt();
39             int q = cin.nextInt();
40             int r = cin.nextInt();
41             int s = cin.nextInt();
42             
43             Arrays.fill(e, 0);
44             addFactorial(p, 1);
45             addFactorial(q, -1);
46             addFactorial(p-q, -1);
47             addFactorial(r, -1);
48             addFactorial(s, 1);
49             addFactorial(r-s, 1);
50             double ans = 1;
51             for(int i = 0; i < prime.size(); i++)
52                 ans *= Math.pow(prime.get(i), e[i]);
53             System.out.printf("%.5f\n", ans);
54         }
55         cin.close();
56     }
57 }